Soil Ecology and Tree Health:  Implications for Management of Urban Forests and
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Soil Ecology and Tree Health: Implications for Management of Urban Forests and Ornamental Landscapes. Dan Herms Department of Entomology The Ohio State University Ohio Agricultural Research and Development Center Acknowledgements: Students and Post-Docs

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Soil Ecology and Tree Health: Implications for Management of Urban Forests and Ornamental Landscapes

Dan Herms

Department of Entomology

The Ohio State University

Ohio Agricultural Research and Development Center


Students and Post-Docs

Jim Blodgett, Rodrigo Chorbadjian, Carolyn Glynn, Bethan Hale, Nate Kleczewski, Joe LaForest, John Lloyd, Marie Egawa


Enrico Bonello, Robert Hansen, Harry Hoitink, Bill Mattson, Ben Stinner

Funding Sources:


USDA National Urban Community Forestry Advisory Council

The Ornamental Landscape as an Ecosystem:

Implications for Pest Management

Herms et al. (1984) J. Arboriculture 10:303-307.

“Understanding the ecological interactions between the biotic and abiotic factors within a landscape enables more effective management of pests.”


Understand how trees allocate their resources in

different environments, and the implications for

the care of trees.


Develop framework based on carbon allocation that can be used to predict tree behavior in different environments.

Conduct experiments to test this framework.

Framework: carbon allocation patterns of trees

Herms, D.A. 2002. Effects of fertilization on insect resistance of woody ornamental plants: reassessing an entrenched paradigm. Environmental Entomology 31:923-933.

Herms, D.A. 2001. Resource allocation trade-offs in trees. Arborists News 10(5):41-47.

Herms, D.A. 2001. Fertilization and pest control. Tree Care Industry 12(5):8-10,12,14.

Herms, D.A. 1998. Understanding tree responses to abiotic and biotic stress complexes. Arborist News 7(1):9-15.

Herms, D.A., and W.J. Mattson. 1997. Trees, stress, and pests. pp. 13-25. In J.E. Lloyd, ed. Plant Health Care for Woody Ornamentals, International Society of Arboriculture, Savoy, IL.

Herms, D.A., and W.J. Mattson. 1992. The dilemma of plants: to grow or defend. Quarterly Review of Biology 67(3):283-335.

Different patterns of resource allocation and acquisition work in different environments

  • Concepts to emphasize:

  • resource acquisition vs. resource allocation

  • carbon budgets and allocation tradeoffs

  • integration of above- and below-ground growth

  • acclimation to stressful environments

Resource acquisition vs. allocation

(Income vs. budgeting)

  • Whole plant carbon budget:

  • photosynthesis rate per unit leaf area

  • total leaf area

  • Allocation tradeoffs: plants have limited resources to support:

  • growth

  • maintenance

  • reproduction

  • storage

  • defense

  • The chemical arsenal of plants: defense and stress tolerance

  • Tannins

  • Phenolic glycosides

  • Terpenes

  • Alkaloids

  • Cyanogenic compounds

  • Defensive proteins

  • Resource allocation patterns

  • In faster growing plants:

  • high allocation to total leaf area

  • high photosynthesis rate

  • lower allocation to root growth

  • lower levels of defensive compounds

  • In slower growing plants:

  • lower allocation to total leaf area

  • high photosynthesis rate

  • higher allocation to root growth

  • higher levels of defensive compounds

Computer-controlled fertigation system to study responses of willow to nutrient availability

Glynn et al. (2007) New Phytologist 176:623-634

  • Nutrient availability and carbon acquisition:

  • no effect on photosynthesis rate / leaf area

  • increased total leaf area

Source / Sink Interactions: carbon moves from sources to sinks via phloem transport

  • Mechanisms of photosynthetic acclimation:

  • Nitrogen allocation and specific leaf mass

  • Root:shoot ratios

Nitrogen deficiency does not cause chlorosis

in plants that have had time to acclimate to

their environment.

Harris, R. W. 1992. Root-shoot ratios. J. Arboriculture 18: 39-42

Stable root:shoot ratios between days 40-85 consistent with equilibrium patterns of resource allocation

Soil fertility and insect resistance:

“Properly fertilized trees are better able to ward off

both insect and disease damage."

“Fertilizing landscape plants promotes their general

health and vitality, making them more resistant to

insect and disease attack."

"Fertilization promotes vigorous growth, disease,

and insect resistance, and stress tolerance."

Fertilization decreased the insect resistance of woody plants in almost every study.

No study showed increased resistance.

Herms, D.A. 2002. Effects of fertilization on insect resistance of woody ornamental plants: reassessing an entrenched paradigm. Environmental Entomology 31:923-933.

Field Studies: Effects of fertilization on paper birch and red pine

Fertilizer Treatment (ANSI standard):

Rate:4.1 lb N / 1000 ft2 / yr

200 kg N / ha / yr

178 lb N / acre / yr

Formulation: 18:5:4 NPK (56 % N slow release)

Timing: early May and mid-Sept (split application)

Fertilization increased growth of Sphaeropsis tip blight lesions by 50%

Blodgett et al. 2005. Forest Ecology and Management 208:273-382.

Effects of nursery fertility regime on crabapple following transplanting

(Lloyd, J.E., et al. 2006. HortScience 41:442-445)

1997: three fertility treatments in container nursery

1998: transplanted to low maintenance landscape.

Greater impact of drought stress on photosynthesis of high fertility plants.

  • Fertilization and stress tolerance:

  • decreased root:shoot ratio

  • increased water requirements

  • decreased secondary metabolites

  • Fertilization decreased drought stress tolerance:

  • Red oak, chestnut oak (Kleiner et al. 1992)

  • American elm (Walters and Reich 1989)

  • Monterey pine (Linder et al. 1987)

  • Red pine (Miller and Timmer 1994)

  • Loblolly pine (Green et al. 1994)

  • Scots pine (Nilsen 1990)

  • Norway spruce (Nilsen 1995)

Nutrient cycling in a forest:

the ultimate slow release fertilizer

Disrupted nutrient cycles in constructed landscapes

Soil quality: the central role of organic matter (SOM)

  • Key determinant of soil structure:

  • oxygen, drainage, water / nutrient holding

  • capacity.

  • Source of essential nutrients for plants.

  • Foundation of soil food web.

  • Continuously depleted and replenished.

The living soil:

In an average cup of healthy soil:

Bacteria: 200 billion

Fungi: 60 miles of hyphae

Protozoa: 20 million

Nematodes: 100,000

Arthropods: 50,000

From: S. Frey, Ohio State University

Most labile N is tied up by microbes

Nutrient Flow: The Central Role of Microbes

Organic Matter

Soil Microbes


Nutrient Cycling in Ornamental Landscapes





Organic N



Microbial Turnover

Mineral N

(NH4,, NO3)




Plant Uptake

  • Key principles of nutrient cycling theory:

  • Microbes are C limited.

  • Plants are N limited.

  • Microbes out-compete plants for N.

  • High C:N organic matter: greater proportion

  • of N immobilized by microbes.

  • Low C:N organic matter: greater proportion

  • of N released (mineralized) by microbes.


Establish general principles for predicting effects of diverse sources of organic matter on soil fertility and plant health.

  • How does mulch affect nutrient availability? :

  • 1. Is the carbon available? Stability of OM

  • 2. Who gets the nitrogen? C:N ratio of OM

Availability of C for microbes: rate of decomposition


Inorganic mulch (stone, shredded tires)

Softwood bark (mature trees)

Softwood bark (immature trees)

Hardwood bark

Ground wood

Wood chips

Composted yard waste


Composted Manure


  • N available for plants determined by net balance between:

  • N mineralization by microbes.

  • N immobilization by microbes.

  • C:N Ratio of OM and Nutrient Availability:

  • C:N ratio > 30:1

  • Microbes N-limited, scavenge N from soil

  • Available N tied up by microbes

  • N available for plants decreases

  • C:N ratio < 30:1

  • N exceeds microbial requirements

  • N release rates increase

  • N available for plants increases

Material C:N RatioRecycled pallets125:1Ground pine bark105:1Fresh wood chips 95:1Hardwood bark 70:1Fresh wood chips w/ foliage 65:1Pine straw 64:1Freshly senesced leaves 55:1Composted wood chips 40:1Composted yard waste 17:1Composted manure 12:1

  • Case study: effects of mulch on soil microbes, nutrient cycling, and plant health.

  • Recycled organic wastes:

  • Composted yard waste (C:N = 17:1)

  • Ground pallets (C:N = 125:1)

Recyled organic

waste as mulch

Experimental Mulches

Composted Yard Trimmings

C:N ratio = 17:1

Ground Wood Pallets

C:N ratio = 125:1

Composted mulch

Ground wood pallets

Experimental approach:

Three Mulch Treatments:

1. Composted yard waste (C:N ratio = 17:1)

2. Ground wood pallets (C:N ratio = 125:1)

3. Bare soil control

Each with and without fertilization

(18-5-4 NPK, 3 lbs N / 1000 ft2 / yr)

Mulch effects on tree growth

Nitrate as signaling molecule: gene expression and regulation of carbon allocation in Arabidopsis:

High soil nitrate:

Up regulation of genes for shoot growth, protein synthesis.

Down regulation of genes for secondary metabolism, root growth.

Low soil nitrate:

Down regulation of genes for shoot growth, protein synthesis.

Up regulation of genes for secondary metabolism, root growth.

Scheible, et al. 2004. Plant Physiology 136:2483-2499.

Zhang and Forde. 2000. Journal of Experimental Botany 51: 51-59.

Hypothesis:trees are adapted to the nutrient fluxes and signals associated with gradual decomposition of leaf litter, including low nitrate levels and high organic N sources.

Can trees be tricked into maladaptive allocation patterns?

Japanese Beetle

Fall Webworm

Trophic cascade from microbes through plants to insect herbivores:

Organic Matter

Microbe Effects on Nutrient Availability

Plant Growth and Defense

Plant-Feeding Insects

  • Conclusions:

  • 1. Both mulches increased:

    • soil organic matter

    • microbial biomass and activity

  • 2. Yard waste increased, ground wooddecreased:

    • nutrient availability

    • plant growth

    • susceptibility to insects

  • How can mulch applied to the soil surface affect nutrient availability below?

  • Soil homogenization by abiotic and

  • biotic forces (rapid increase in SOM in

  • mulched plots)

  • Subterranean foraging by hyphae of

  • fungi that have colonized the mulch.

  • 3. It just does.

  • Consistent with hypotheses:

  • 1. Soil microbes are carbon-limited.

  • 2. Plants are nitrogen limited.

  • Microbes out-compete plants for nitrogen.

  • 4. Competition for N mediated by C:N ratio of OM.

  • 5. Trade-off between growth and defense in plants.

  • Prescription mulching:

  • Low C:N mulch (e.g. composted yard trimmings):

  • degraded soils

  • increased plant growth

  • new landscapes

  • High C:N mulch (e.g. recycled pallets):

  • slow to moderate growth

  • established plantings

Mulch volcanoes are not good for trees!

If you must make volcanoes, at least keep the mulch in the bags

Ecological Interactions in Sub-Soil

Comparison of Sub-Soil and Top Soil Plots:

Sub Soil Top Soil

Organic Matter (%) 0.75 2.24

Clay (%) 24 17

Total N (ppm) 5601790

Nitrate N (ppm) 8 161

Phosphorus (ppm) 8 50

In subsoil:

Fertilization increased growth and decreased phenolic compounds.

In topsoil:

Fertilization had no effect on growth or phenolics.

Fertilization effects on fall webworm

  • Mycorrhizae research:

  • Root colonization in subsoil

  • Effects of fertilizer

  • Interactions between native and commercial

  • mycorrhizae

Nate Kleczewski

  • Allocation to mycorrhizae

  • Benefits:

  • phosphorus acquisition

  • organic nitrogen uptake

  • increased drought tolerance

  • increased resistance to root disease

  • Costs:

  • up to 40% of carbon assimilated by the plant.

Symbiosis (living together):

mutualism parasitism

Under some conditions, mycorrhizal fungi act as parasites, taking more than they give.

Plants can suppress mycorrhizae when costs exceed benefits:

High nutrient availability

(should mycorrhizal spores be applied with fertilizer?)

Low carbon availability (e.g. shade, defoliation)

Questions regarding commercial mycorrhizae:

  • Will they establish?

  • Will they compete with native mycorrhizal fungi?

  • Will they enhance plant growth and survival?

Key Findings:1. Only native EMF were detected.2. No difference between top-soil and sub-soil.


* Estimated number of viable propagules

Key Findings:3. High fertility suppressed mycorrhizae.


Key Findings:1. Only native EMF were detected.2. No difference between top-soil and sub-soil.3. High fertility suppressed mycorrhizae.

  • Conclusions:

  • Increased soil fertility:

  • Increases growth

  • Decreases chemical defenses

  • Decreases root:shoot ratio

  • Can decrease pest resistance

  • Can decreases drought stress tolerance

  • Effects are independent of nutrient source, form, or timing:

  • container fertigation

  • conventional fertilization (ANSI guidelines)

  • stored nutrients obtained the previous year

  • mulch effects on soil microbes and nutrient cycling

  • inherently fertile vs. infertile soil.

  • This doesn’t mean:

  • fertilization is bad.

  • fertilization will increase pest problems in landscapes

  • (these studies haven’t been done yet).

This does mean:

The data do not support the conventional wisdom that fertilization increases pest resistance.

  • The Natural Tree Environment:

  • nutrient limited soils

  • frequent episodes of drought stress

  • insects and pathogens

  • The Natural Tree Response:

  • high root:shoot ratios

  • high levels of storage carbohydrates

  • high levels of defensive chemicals

  • moderate growth

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